In coal-fired steam generators, the manner in which the ash is handled and disposed of is an item of considerable importance. One means used today for continuously removing ash and slag which falls through an opening in the furnace bottom is a scraper conveyor which is submerged in a tank of water. When the unit is shut down for periodic maintenance, it is desirable to be able to move the entire scraper conveyor unit, including the tank in which it is housed, from beneath the furnace so that it can be easily worked on, and so that it can be replaced with another unit if major repair is required.
A water seal is generally provided for preventing the atmosphere from being exposed to the furnace interior. This consists of a plate extending down and surrounding the furnace bottom opening, which plate extends into the submerged scraper conveyor tank, which tank is filled with water, thus forming a water seal. This type of seal is provided because it allows the furnace to grow relative to the submerged scraper conveyor unit caused by thermal expansion. Large furnaces are generally top supported, so that they are free to expand in a downward direction when the unit is first started up. This growth can be on the order of 10-12" from the cold to the hot condition.
The above provides the problem of how to be able to quickly remove the submerged scraper conveyor unit from beneath the furnace when the unit is shut down for maintenance. In the past, it has been necessary to allow the unit to cool down to a point where workmen can unbolt the seal plate from the furnace bottom. This cooling and unbolting time is considerable and can cause the entire steam generator to be down for lengthy maintenance periods.